In semiconductor manufacture, micro lithography is used in the formation of integrated circuits on a semiconductor wafer. During a lithographic process, a form of radiant energy such as ultraviolet light, is passed through a mask or reticle and onto the semiconductor wafer. The mask contains opaque and transparent regions formed in a predetermined pattern. A grating pattern, for example, may be used to define parallel spaced conducting lines on a semiconductor wafer. The ultraviolet light exposes the mask pattern on a layer of resist formed on the wafer. The resist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. The patterned resist can then be used during a subsequent semiconductor fabrication process such as ion implantation or etching.
As microcircuit densities have increased, the size of the features of semiconductor devices have decreased to the sub micron level. These sub micron features may include the width and spacing of metal conducting lines or the size of various geometric features of active semiconductor devices. The requirement of sub micron features in semiconductor manufacture has necessitated the development of improved lithographic processes and systems. One such improved lithographic process is known as phase shift lithography.
With phase shift lithography the interference of light rays is used to overcome diffraction and improve the resolution and depth of optical images projected onto a target. In phase shift lithography, the phase of an exposure light at the object is controlled such that adjacent bright areas are formed preferably 180 degrees out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference. This technique improves total resolution at the object and allows resolutions as fine as 0.10 .mu.m to occur.
In general, a phase shifting photomask is constructed with a repetitive pattern formed of three distinct layers or areas. An opaque layer provides areas that allow no light transmission, a transparent layer provides areas which allow close to 100% of light to pass through and a phase shift layer provides areas which allow close to 100% of light to pass through but phase shifted 180 degrees from the light passing through the transparent areas. The transparent areas and phase shift areas are situated such that light rays diffracted through each area is canceled out in a darkened area there between. This creates the pattern of dark and bright areas which can be used to clearly delineate features of a pattern defined by the opaque layer of the mask on a photopatterned semiconductor wafer.
Recently, different techniques have been developed in the art for fabricating different types of phase shifting photomasks. One type of phase shifting mask, named after a pioneer researcher in the field, M. D. Levenson, is known in the art as a "Levenson" phase shifting mask. This type of mask is also referred to as an "alternating aperture" phase shifting mask because opaque light blockers alternate with light transmission apertures and every other aperture contains a phase shifter.
This type of mask is typically formed on a transparent quartz substrate. An opaque layer, formed of a material such as chromium, is deposited on the quartz substrate and etched with openings in a desired pattern. Phase shift areas on the mask are formed by depositing a phase shift material over the opaque layer and into every other opening in the opaque layer. This is termed an "additive" phase shifting mask. Alternately, phase shift areas of the mask may be formed in areas of the quartz substrate having a decreased thickness. This is termed a subtractive phase shifting mask.
Two types of Levenson phase shifting photomasks are shown in FIGS. 1A and 1B. FIG. 1A shows an additive phase shifting mask 8A comprising a transparent substrate 10A and an opaque layer 12A having a pattern of etched openings 16A. The phase shifters 14A for the phase shifting mask 8A are formed as segments of a light transmissive material, such as SiO.sub.2, deposited in every other opening 16A in the opaque layer 12A.
FIG. 1B shows a subtractive phase shifting mask 8B. In a subtractive phase shifting mask 8B, the phase shifters 14B are formed by etching the substrate 10B aligned with every other opening 16B in the opaque layer 12B. In the subtractive phase shifting mask 8B, the unetched portions of the substrate form the phase shifters 14B. Although the additive and subtractive phase shifting masks are fabricated by different methods, the operation of these masks is equivalent.
Another type of phase shifting mask is known as a rim phase shifting mask. A rim phase shifting mask includes phase shifters that are formed on the edges of the opaque light blocking elements. A rim phase shifting mask can also be made using an additive process or a subtractive process.
In FIG. 2A, a rim phase shifting mask 18A is formed by an additive process and includes a transparent substrate 20A, opaque light blockers 22A and light transmissive openings 24A. Rim phase shifters 26A are formed on the edges of the features defined by the opaque light blockers 22A. The rim phase shifters 26A are formed by depositing a layer of a transparent material such as silicon dioxide (SiO.sub.2) to a predetermined thickness.
In use of the rim phase shifting mask 18A, the light passing through a rim phase shifter 26A is phase shifted relative to the light passing through a light transmission opening 24A. This is because the light passing through a rim phase shifter 26A must pass through a thicker section of the transparent phase shift material. The phase shifted light forms a null on the wafer that corresponds to the edges of the opaque light blockers 22A. This overcomes the effects of diffraction along the edges of the opaque light blockers 22A and produces a sharpened image.
The thickness "t" of the rim phase shifters 26A is selected to achieve a phase shift of 180.degree. (.pi.) or an odd whole multiple of 180.degree.. This thickness "t" can be determined using the formula: EQU t=i.lambda./2(n-1)
where
t=thickness of rim phase shifters PA2 i=an odd integer PA2 .lambda.=wavelength of exposure light PA2 n=refractive index of phase shift material at the exposure wavelength.
FIG. 2B illustrates a rim phase shifting mask 18B formed using a subtractive process. The rim phase shifting mask 18B includes a transparent substrate 20B, opaque light blockers 22B and light transmissive openings 24B. Rim phase shifters 26B are formed on the edges of the opaque light blockers 22B by etching the substrate 20B to a depth of "d". The depth "d" corresponds to the thickness "t" in the above formula.
One problem that occurs in the manufacture of rim phase shifting masks is that it is difficult to align the rim phase shifters to the edges of the features. This is because different photomasks are required to form the opaque light blockers and the rim phase shifters. These two different photomasks can be difficult to accurately align at the submicron feature sizes required by VLSI and ULSI semiconductor manufacture. In addition, it is sometimes difficult to control the thickness "t" or depth "d" of the rim phase shifters.
In view of the foregoing, it is an object of the present invention to provide an improved method for fabricating phase shifting masks for semiconductor manufacture. It is a further object of the present invention to provide an improved method for fabricating rim phase shifting masks in which rim phase shifters are self-aligned with the edges of an opaque light blocker. It is yet another object of the present invention to provide an improved method for fabricating rim phase shifting masks that is simple, low cost and adaptable to large scale semiconductor manufacture.
Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.